1. Field of the Invention
The present invention relates to a bicycle derailleur including a parallelogram link mechanism having a base member connected to a bicycle frame, a support member for supporting a chain shifting member, and a pair of pivotal links extending between the support member and base member. The support member is movable relative to the base member in one direction between a smaller gear and a larger gear under a pulling force of a control cable, and in the other direction under a restoring force of a return spring.
2. Description of the Related Art
A conventional bicycle derailleur as noted above is disclosed in Japanese Patent Publication Kokai No. 61-12486, for example. This derailleur includes a control member pivotally connected to one of a pair of pivotal links, and a torsion spring mounted on a support axis supporting the control member. The torsion spring has one leg thereof engaged with the control member, and the other leg engaged through a pivotal member with the other pivotal link opposed to the pivotal link pivotally supporting the control member. When a control cable connected to the control member is pulled, its pulling force is transmitted to the pivotal links through the torsion spring. Then, the pivotal links pivot relative to the base member toward a larger gear, with the support member moving relative to the base member at the same time. As a result, a guide wheel moves relative to the base member to shift a chain to a larger one of a plurality of gears. Conversely, when the control cable is relaxed, the pivotal links pivot relative to the base member toward a smaller gear under the elastic restoring force of the torsion spring, with the support member moving relative to the base member at the same time. As a result, the guide wheel moves relative to the base member to shift the chain to a smaller one of the gears.
When a shifting operation is carried out to shift the chain to a larger gear staying still, a resistance stronger than usual is applied to the chain shifting. Consequently, when the torsion spring is elastically deformed by the pulling force of the control cable, only the control member pivots relative to the pivotal links, with the support member remaining still. Subsequently, when the gears begin rotating, the resistance to the chain shifting falls to a normal value. Only then, the pivotal member moves to a position opposed to the control member, which is due to an operating force of the torsion spring stored therein by its preceding elastic deformation. With the resultant movement of the support member, the guide wheel moves relative to the base member to shift the chain to a selected larger gear.
That is, the torsion spring acts as a return spring to effect an operation to shift the chain to a smaller gear. During an operation to shift the chain to a larger gear, depending on the degree of a resistance to the operation, the torsion spring acts as a saver spring to store the operating force.
In the prior construction noted above, however, the coiled portion of the spring may be rubbed when the spring exerts an operating force in a shifting operation. This results in a situation where an operating force corresponding to a resilience intrinsic to the spring itself is not applied to actual chain shifting. Further, earth or sand lodged in or adhering to the coiled portion is not easy to remove, which also tends to cause loss of resilience. As a countermeasure to such phenomena, a strong spring which can cover some loss of resilience is employed to achieve chain shifting as desired. This results in a relatively strong force required when effecting a shifting operation against the force of the spring.